Laser wire bonding for wire embedded dielectrics to integrated circuits

ABSTRACT

A method and apparatus for connecting a lead of a lead frame to a contact pad of a semiconductor chip using a laser or other energy beam is herein disclosed. The lead may be wire bonded to the contact pad by heating the ends of a wire until the wire fuses to the contact pad and lead or an energy-fusible, electrically-conductive material may be used to bond the ends of the wire to the contact pad and lead. In addition, this invention has utility for both conventional lead frame/semiconductor chip configurations and lead-over-chip configurations. In addition, with a lead-over-chip configuration, the lead may be directly bonded to the contact pad with a conductive material disposed between the lead and the contact pad.

This is a division of application Ser. No. 08/654,192, filed May 28,1996, now U.S. Pat. No. 5,731,244.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to wire bonding lead frames tosemiconductor dice, and more specifically to wire bonding a lead frameto a semiconductor die using a laser beam to provide the energynecessary to bond or fuse a wire to a lead of a lead frame and to acontact pad of a semiconductor die. The method and apparatus oflead-to-chip bonding herein described is applicable to either aconventional lead frame and chip arrangement or a lead-over-chip (LOC)arrangement, in any instance, where the lead of a lead frame is directlyor indirectly bonded to the contact pad of a semiconductor chip.

2. State of the Art

Various types of semiconductor chips are connected to lead frames andsubsequently encapsulated in plastic for use in a wide variety ofapplications. A conventional lead frame is typically formed from asingle continuous sheet of metal, typically by metal stampingoperations. The conventional lead frame includes an outer supportingframe, a central semiconductor chip supporting pad and a plurality ofleads, each lead having, in turn, a terminal bonding portion near thecentral chip supporting pad. Ultimately, the outer supporting frame ofthe lead frame is removed after the wire bonds between the contact padsof the semiconductor chip and the leads are made and the semiconductorchip and lead frame have been encapsulated.

In an LOC lead frame, the lead frame has no central chip supporting padwith the semiconductor chip being held in position with respect to thelead frame and leads by means of adhesive strips secured to the leads ofthe lead frame and the semiconductor chip.

A typical apparatus and method for forming the wire bonds between thecontact pads on semiconductor chip and the leads of lead frames isillustrated in U.S. Pat. No. 4,600,138. As disclosed, a bond head isshown moving from a first bonding location to a second bonding location.The end of the wire is bonded to the first bonding location by the bondhead. The bond head moves vertically away from the first bondinglocation to draw a length of wire necessary to make the wire bond. Thebond head is then moved to the second bonding location with subsequentbonding of the wire to the second bonding location. The bond head isthen used to pull and subsequently break away the remaining wire fromthe second bonding location. The bond head is then ready to be moved toanother first bonding location for affecting another wire bond.

Typically, the bond head is heated to assist the formation of the wirebond. The heat and subsequent pressure applied by the bond head fusesthe end of the wire to the contact pad. Ultrasonic vibration inconjunction with a heated bond head may also be used to affect a wirebond. Typically, there is a single bond head for making all of the wirebonds of the semiconductor chip. As should be recognized by thoseskilled in the art, such an operation is inherently mechanical in natureand thus limited to the speed of movement of the mechanical device.

One method of speeding a conventional wire bonding process is to providethe heat necessary to affect a wire bond by utilizing heat generatedfrom a laser beam to heat the bond head. Such apparatuses are disclosedin U.S. Pat. Nos. 4,534,811 to Ainslie et al., and 4,845,354 to Gupta etal. However, as the number of connections per semiconductor chipincrease and the size of the leads decrease, such a bonding tool becomesimpractical.

It has also been recognized in the art to use laser beams to form alead-to-chip bond. For example, a method for reflowing solder to bond anelectrical lead to a solder pad using a laser, in which the solder pad,rather than the terminal, is irradiated by the laser beam, is disclosedin U.S. Pat. No. 4,926,022 to Freedman. In addition, in U.S. Pat. No.5,274,210 to Freedman et al., electrical connections may be made bycoating conductive elements with a non-flux, non-metallic, coatingmaterial making it possible to use a laser for bonding. The laser iseither moved in a continuous sweep around all of the connections orpulsed.

It has also been recognized in the art to use a laser beam to bond thebumps of an integrated circuit to a tape automated bonding (TAB) tapelead. TAB, in general, has been one attempt in the art to increase thespeed and efficiency of the chip-to-lead bonding process. For example,in U.S. Pat. Nos. 4,978,835 to Luijtjes et al. and 5,049,718 and5,083,007 to Spletter et al., a laser beam is directed onto the ends ofthe leads of a TAB tape.

None of the previously mentioned prior art references, however, havesuccessfully utilized laser light to reduce the mechanical limitationsof the bonding process. More specifically, prior art devices either movethe device relative to the laser for every bond, or a single laser beamto every bonding site. Thus, it would be advantageous to provide anapparatus and method for forming wire bonds using a laser in which thelaser need not move for each bond and where more than one bond can bemade substantially simultaneously.

SUMMARY

Accordingly, the present invention provides a bonding apparatus andmethod of using the same for bonding any lead frame, either aconventional lead frame or a lead-over-chip (LOC) lead frame, to asemiconductor chip. Preferably, the semiconductor chip will include atleast one contact pad on its active surface for providing an output, orinput as the case may be, of the chip. Likewise, the lead frame willinclude at least one lead to be connected to the contact pad of thechip. The chip/lead frame arrangement may be one where wire bonding isnecessary to make the electrical connection between the contact pad andthe lead, or an LOC arrangement where the lead of the lead frame extendsover the active surface of the chip and is bonded to the contact padwith a short wire or a bump of solder. In either case, the bond requiredto make the connection uses an energy beam from a beam-emitting energysource to provide the energy necessary to make the connection.

In a chip/lead frame arrangement where a wire bond is used to make theelectrical connection, the wire is aligned with the contact pad and thelead is subsequently bonded or fused to each using a directed energybeam. A beam of energy is focused on the site of the bond with a lens orplurality of lenses. Moreover, the wire may be directly bonded or fusedto the contact pad and lead by melting the wire with the energy beam, oran energy fusible, electrically conductive bonding material may beprovided proximate the bonding site.

In a preferred embodiment, the wire bonds at the contact pad and at thelead are substantially simultaneously bonded or fused. This may beaccomplished by using more than one beam emitting energy source todirect more than one beam of energy, each to a different bonding site,or providing a single beam emitting energy source and splitting the beaminto more than one smaller beams and directing the smaller beams todifferent bonding sites.

The beam emitting energy source used in conjunction with the presentinvention may be of various types known in the art. For example, theenergy source may emit a laser beam, such as that produced by a pulsedsolid state laser, a carbon dioxide laser, a Nd:YAG lasers, or a Nd:YLFlaser, a focused beam of light, a beam of radiant energy such as anelectron beam, or a heat source, etc. In any case, the energy beam ispreferably directed to the bonding site by prisms, mirrors, fiberoptics, lenses and/or other reflective and/or deflective surfaces orcombinations thereof. More specifically, in one preferred embodiment,each beam of energy is directed by prisms or mirrors from the energysource toward each bonding site. At least one lens is provided betweeneach bonding site and the prisms or mirrors to further focus the beam ofenergy onto the bonding site. Each lens may be individually supported bya frame-like support structure or contained within an optical flat inwhich a plurality of lenses are formed. For a frame-like structure, thelenses may be moved and/or oriented to accommodate different chip/leadframe configurations. If the lenses are contained in an optical flat, adifferent optical flat may be used to accommodate various chip/leadframe configurations. In addition, the reflective and/or deflectivesurface may be articulatable to direct the energy beam to variousbonding sites. With such an articulatable configuration, variousconfigurations of lead frames and chips can be accommodated with thepresent invention.

In use, the energy beams are directed to a first set of bonding sitesuntil the heat generated from the energy beams create the bonds or fusethe bonds (i.e., wire bond or LOC bond) associated with the first set.The apparatus then translates the energy beams relative to the chip to asecond set of bonding sites to make a second set of bonds. This processis repeated until all of the bonds associated with the chip/lead framearrangement are formed.

An indexing system may also be associated with the apparatus to indexchips, lead frames and other components into and out of the bondinglocation. The indexing system may comprise conveyors, articulating arms,magazines for housing the semiconductor device components, and otherequipment known in the art. In addition, the entire system, fromcontrolling the operation of the energy source to controlling which setof bonding sites are bonded to indexing the semiconductor devicecomponents, is controlled by at least one or more microprocessors.

As previously mentioned, a semiconductor chip bonded to its associatedlead frame in accordance with the present invention may have aconventional configuration where the contact pads are positionedproximate the periphery of the chip or a LOC configuration where thecontact pads are positioned closer to a center line of the chip. Ineither case, in a preferred embodiment, a thin flexible dielectricmaterial (foil) containing fully or partially embedded wires may beplaced at least partially over the surface of the chip containing thecontact pads and the leads of the lead frame. When properly positioned,the wires extend from the contact pads to the leads of the lead frame.The foil may be adhesively attached to the chip and/or lead frame, heldin place by a slight vacuum, or retained by a suitable clamping devicein order to maintain proper alignment of the wires relative to the chipand lead frame. At the ends of the each wire, an energy bondable,fusible, electrically conductive material (such as solder) may beprovided for bonding the ends of the wire to the semiconductor chip andlead frame. Similarly, the energy bondable, fusible, electricallyconductive material may be attached to the contact pads of the chipand/or the leads of the lead frame prior to positioning of the foil suchthat the energy fusible, electrically conductive material may be heatedand subsequently bonded to the ends of each wire.

For an LOC configuration where the leads of the lead frame extend overthe contact pads, an energy bondable, fusible, electrically conductivematerial (e.g., solder) may be provided between the lead and the contactpad. The solder may be bumped on to the contact pads by methods known inthe art or attached to the ends of the leads to define a protuberance onthe end of the lead so that when the lead frame is superimposed over thechip, the protuberance of solder is positioned above each contact pad.The solder may be bonded or fused to make the electrical connectionbetween the leads and the contact pads by heating the leads themselveswith an energy beam or providing leads that define openings throughwhich the beam may be directed directly onto the solder. In yet anotherpreferred embodiment, the ends of the leads themselves may be configuredto contact the contact pads and may be bonded directly thereto byheating the lead.

A preferred embodiment of a semiconductor device manufactured accordingto the present invention would comprise a semiconductor chip having aplurality of contact pads, a lead frame having a plurality of leads, afoil layer or other suitable type material having a plurality of wiresat least partially embedded therein, and a laser-bondable,electrically-conductive material making the electrical connectionsbetween the wires and the contact pads and leads.

Although the bonding apparatus of the present invention has beendescribed in relation to several preferred embodiments, it is believedthat a major advantage of the apparatus according to the presentinvention is the efficient use of a beam emitting energy source, such asa laser, to quickly and efficiently bond a lead frame to a semiconductorchip by reducing the mechanical movements generally associated withprior art bonding apparatuses. This and other features of the inventionwill become apparent from the following detailed description taken inconjunction with the accompanying drawings and as defined by theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a first embodiment of thelaser-bonding apparatus according to the present invention;

FIG. 2 is a schematic top view of an optical flat use in thelaser-bonding apparatus in FIG. 1;

FIG. 3A is a schematic bottom view of a wire-embedded foil according tothe present invention;

FIG. 3B is a schematic side view of a wire-embedded foil according shownin FIG. 3A;

FIG. 4 is a schematic side view of a LOC configuration according to thepresent invention;

FIG. 5 is a schematic side view of a second embodiment of thelaser-bonding apparatus according to the present invention;

FIG. 6 is a partial top view of a second embodiment of a lead of an LOClead frame shown in FIG. 5;

FIG. 7 is a schematic side view of a third embodiment of thelaser-bonding apparatus according to the present invention;

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

As illustrated in FIG. 1, the laser-bonding apparatus 10 is comprised ofa plurality of lasers 12, 14, 16, and 18 emitting laser beams 20, 22,24, and 26, respectively. The laser beams 20, 22, 24, and 26 aredirected toward lenses 28, 30, 32, and 34, respectively, by prisms 36,38, 40 and 42. The lenses 28, 30, 32, and 34 focus the laser beams 20,22, 24, and 26 onto the bonding sites 44, 46, 48, and 50, respectively,associated with the semiconductor chip 52 and lead fingers or leads 54and 68 of a lead frame. The lenses 28, 30, 32, and 34 are formed in aframe-like structure or an optical flat 56 above each bonding site 44,46, 48 and 50.

The optical flat 56 as shown in FIG. 2 has a plurality of lenses 57, 59,61, and 63, such as lenses 28, 30, 32 and 34, formed in longitudinalrows along the length of the optical flat 56 corresponding to thebonding sites of a chip 52 and lead fingers 54 and 68, such as bondingsites 44, 46, 48, and 50. As illustrated by the arrow and bar 65, thelaser beams 20, 22, 24 and 26 are incrementally moved across the opticalflat from one set of leases 57, 79, 61, and 63 to the next until all ofthe bonds have been fused located on a particular chip 52 and leadfingers 54 and 68.

The semiconductor chip 52 and lead fingers 54 and 68 are supported on aplatform or chip support 58. The chip support 56 may be heated so thatheat generated by the laser beams 20, 22, 24, and 26 at the bondingsites 44, 46, 48, and 50 do not create such an extreme point oflocalized heating that could stress the chip 52 and/or the lead fingers54 and 68. In addition, the chip support 58 may include structure as isknown in the art to help align the lead fingers 54 and 68 relative tothe chip 52 and the chip 52 relative to the rest of the laser-bondingapparatus 10.

In order to secure and align the wires 60 necessary to make wire bondsbetween the chip 52 and the lead fingers 54 and 68, as illustrated inFIGS. 3A and 3B, a foil, relatively-thin dielectric material, or othersuitable material 62 may be used to support a plurality of wires 60. Thewires 60 may be fully or partially embedded in the foil 62 so long asthe foil 62 can maintain the relative positions of the wires 60. At theend of each wire 60, an energy-bondable, fusible,electrically-conductive material 64, such as solder or other materialknown in the art, may also be provided to make the bond between thewires 60 and the bond sites 44, 46, 48, and 50. A flux may also beapplied to the ends 66 of the wires 60 to help the wires 60 bond to thebonding sites 44, 46, 48, and 50, whether an energy-bondable, fusible,electrically conductive material 64 is used or not.

As shown in FIGS. 1 and 3B, the foil 62 is contoured to fit over thechip 52 and extend down to the lead fingers 68 of the lead frame 54.This contoured shape may be formed into the foil by bending the foil tocorrespond to the shape of the chip 52/lead fingers 54 and 68configuration or may take this shape due to the foil's 62 flexiblenature. Because the foil 62 is relatively thin and flexible, it may benecessary to retain the foil 62 relative to the chip 52 and lead fingers54 and 68 during the bonding process. Retaining the foil 62 may beaccomplished by applying an adhesive to the underside 70 so that thefoil 62 may be adhesively bonded to the chip 52 and/or the lead fingers54 and 68. In addition to or in lieu of adhesive retaining, the foil 62may be secured during bonding by retaining members 72 and 74 that holdthe foil 62 relative to the lead fingers 54 and 68 and a resilient pad76 that holds the foil 62 in place relative to the chip 52. Moreover,the foil 62 may be retained by providing a slight vacuum to theunderside 70 of the foil 62 to draw the foil 62 onto the leads 68 andthe chip 52.

In addition, to a conventional chip 52/lead fingers 54 and 68arrangement as illustrated in FIG. 1, the laser-bonding apparatus 10 ofthe present invention can also be used to wire bond a LOC arrangement,as illustrated in FIG. 4, where the leads 80 of the lead frame 82 extendover the active surface 84 of the chip 86. Typically, such a chip 86will have a plurality of contact pads 88 proximate the center of thechip 86. Thus, in order to shorten the length of the wires 90 necessaryto make an electrical connection between the lead 80 and the contact pad88, the leads 80 extend over the active surface 84 proximate the bondpads 88. A foil 92 containing wires 90 may also be used to house andsupport the wires 90 in a similar manner to the foil 62 described inrelation to FIGS. 3A and 3B. In addition, retaining members 94, 96, and98 may also be incorporated into the bonding apparatus 10 to retain thefoil relative to the chip 86 and lead frame 82 during the bondingprocess.

In an alternate embodiment of the laser-bonding apparatus 100 depictedin FIG. 5, a LOC arrangement 101 is being bonded using a single laser102. In this LOC arrangement 101, however, as opposed to thatillustrated in FIG. 4, the lead fingers or leads 104 and 105 of the leadframe 112 are being bonded directly to the contact pads 106 and 107,respectively, of the chip 108. In order to substantially simultaneouslybond the leads 104 and 105 to the contact pads 106 and 107,respectively, the laser beam 110 is split by a beam splitter 114, as isknown in the art. The two beams 116 and 118 are directed to focusinglenses 120 and 122 by mirrors or prisms 124 and 126. The focusing lenses120 and 122 focus the beams 116 and 118 onto the bonding sites 128 and130. The lenses 120 and 122 may be moved to accommodate variouschip/lead frame configuration and/or articulatable to direct the beams116 and 188 to various bonding sites. The prisms 124 and 126 may also bemovable and/or articulatable in the x-axis, y-axis, and z-axis.

Because the passivation layer 132 of the chip 108 typically extendsabove the contact pads 106 and 107, in order to make contact with theleads 104 and 105, either a protuberance or other extension must beprovided on the leads 104 and 105 or the contact pads 106 and 107 mustbe raised at least to the level of the passivation layer 132. If afiller material 134 is used, the filler material 134 should beconductive to provide an electrical path between the contact pads 106and 107 and the leads 104 and 105. Moreover, the filler material 134must be bondable or fusible by the energy provided by the beams 116 and118. In addition, the leads 104 and 105 should be held in positionrelative to the passivation layer 132 by a retainer, such as clamps 142and 144. The chip 108 may also be held in position by a recess 146defined by the chip support 148 sized and shaped to securely hold thechip 108 in place during bonding.

When bonding or fusing the leads 104 and 105 to the contact pads 106 and107, respectively, the top surfaces 136 and 138 of the leads 104 and105, respectively, may be heated by the beams 116 and 118, or, asillustrated in FIG. 6, an aperture or opening 140 can be provided ineach of the leads, such as lead 104, to expose the filler material 134directly to the beam 116.

Referring now to FIG. 7, a plurality of fiber optics 150, 152, 154, and156 are used to direct the laser beams 158, 160, 162, and 164 emanatingfrom the lasers 166, 168, 170 and 172, respectively. That is, as will berecognized by those skilled in the art, there may be other ways known inthe art to direct the laser beams 158, 160, 162 and 164 from the lasers166, 168, 170 and 172 to the lenses 174, 176, 178 and 180.

In all of the preferred embodiments of bonding apparatus according tothe present invention, the manipulation of the lasers as well as theindexing of chip components, such as the foil, die and lead frames, canbe automated and controlled by one or more microprocessors 200 as isknown in the art.

It should be noted that the laser source is preferably any high-power,pulsed, solid state or continuous wave laser, such as Nd:YAG, Nd:YLF,Ar-ion, CO₂, Cu vapor, or other suitable lasers known in the art, or afocused beam of light or a beam of energy or radiant energy, such as anelectron beam or heat source. It should be recognized by those skilledin the art that the apparatus according to the present invention may beused on any semiconductor chip and associated lead frame having eitherconventional configurations as is known in the art or a specializedarrangement. Those skilled in the art will also appreciate that thenumber of lasers and beams therefrom may be increased or decreaseddepending on the number of wire bonds to be formed at substantially thesame time. Further, the invention may be practiced on many semiconductordevices where wire bonding or LOC bonding is desired such as bonding achip to a printed circuit board. Thus, the terms "chip" and "lead frame"as used herein are intended as exemplary and not limiting, the inventionhaving applicability to any semiconductor-related structure employing awire bond or a LOC-type bond. It will also be appreciated by one ofordinary skill in the art that one or more features of any of theillustrated embodiments may be combined with one or more features fromanother to form yet another combination within the scope of theinvention as described and claimed herein. Thus, while certainrepresentative embodiments and details have been shown for purposes ofillustrating the invention, it will be apparent to those skilled in theart that various changes in the invention disclosed herein may be madewithout departing from the scope of the invention, which is defined inthe appended claims.

What is claimed is:
 1. A semiconductor device, comprising:at least onesemiconductor die having a plurality of contact pads located on asurface thereof; at least one lead frame having a plurality of leadspositioned proximate said at least one semiconductor die; and at leastone foil layer having a plurality of wires at least partially embeddedtherein, a plurality of said plurality of wires connected between acorresponding plurality of said plurality of leads and a correspondingplurality of said plurality of contact pads forming a correspondingplurality of wire bonds thereinbetween.
 2. The semiconductor device ofclaim 1, further including a laser-bondable, electrically-conductivematerial bonded to each end of said plurality of said plurality ofwires.
 3. The semiconductor device of claim 1, further including alaser-bondable, electrically-conductive material fused to each end ofsaid plurality of said plurality of wires.